The present invention relates to moisture-curing polyurethane-based hot-melt compositions with enhanced non-sagging.
Hot-melt adhesive compositions based on thermoplastic polymers are well known. Usually, they are applied at temperatures of about 150.degree.-200.degree. C. and solidify by cooling down. This leads to fast setting bonds but with poor temperature resistance.
Moisture-curing hot-melt adhesive compositions based on polyurethane prepolymers are also well known. They are usually applied at temperatures between 90 and 160.degree. C. Curing with environmental moisture leads to bonds with superior thermal and hydrolytic stability.
All these hot-melt adhesive compositions are low viscous liquids at application temperature because of the requirement for workability of adhesive and effective wetting of substrates.
Problems may occur when applying larger beads or thicker layers of such compositions, particularly with vertical applications, when the melt tends to flow down or spread uncontrollably before solidification takes place.
Accordingly, it is the object of the present invention to provide a moisture-curing polyurethane-based hot-melt composition with improved sag resistance or high structural viscosity, i.e. with high viscosity at low shear rate and low viscosity at high shear rate.
It has been proposed to incorporate thixotropic agents in one-part moisture-curing polyurethane adhesive or sealant compositions.
European patent application 0.244.608 discloses a reactive hot-melt adhesive containing thermoplastic isocyanate binder and thixotropic agent.
Several thixotropic agents are disclosed, however, without detailed specification. However, the disclosed composition can still de deformed visco-plastically after cooling.
Our co-pending UK patent application 8829524.1 discloses a hot-melt glue-stick comprising at least one ethylene vinyl acetate copolymer characterised by the inclusion of 1-15 wt%, preferably 5-8 wt%, of fumed silica.